1. Field of the Invention
The present invention is related to an antenna and particularly to an antenna with double groundings.
2. Description of Related Art
In view of the research of wireless communication, different systems have diversified frequencies and bandwidths, and require different designs of radiation patterns and polarizations of the wave radiated by the antenna from time to time. In addition, the environment of mobile communication is full of interference and variables. An antenna not only needs to coordinate the frequency, bandwidth, pattern of radiation wave, and polarization, but is required to overcome problems, such as interference of multipath, transition of radiation wave polarization, change of radiation wave pattern, and size, weight, and shape of the antenna. Among the above, the interference of multipath causes fading during the transmission of signal and greatly reduces the reliability of the wireless communication system.
At present, the means to overcome the fading problem is to use spatial diversity, pattern diversity, and polarization diversity of the antenna. In view of the design, a circularly polarized antenna has no particular polarization direction when receiving or transmitting radiation waves, and thus is able to overcome the influence of phase difference resulting from multipath interference. For this reason, antennas of satellite communication systems, global positioning systems, microwave AV monitor systems, electronic charging systems, microwave remote control and microwave measuring systems, and so forth all adopt circular polarization design for transmitting signals.
FIG. 1A and FIG. 1B respectively illustrate a structural view and a radiation wave pattern of a conventional circularly polarized antenna. The conventional circularly polarized antenna has a radiating patch 110 printed on a ceramic substrate 120 which is disposed on a symmetrical ground plane 130. The “symmetrical” mentioned here indicates that the distance from the periphery of the ceramic substrate 120 to the periphery of the ground plane 130 is equal. The radiating patch 110 is basically a square metal plane. An isosceles triangle is respectively cut off from a lower left corner and a upper right corner of the square metal plane, and the path difference thereof is based on to determine the properties of the radiation wave. When the conventional circularly polarized antenna in FIG. 1A is placed on the symmetrical ground plane 130, a radiation pattern as illustrated in FIG. 1B is generated, wherein the largest gain of the radiation wave is along a +z direction (a vertex direction).
However, in the current communication products, the design of the circularly polarized antenna, which has a structure of FIG. 2A, is usually limited to the shapes and systems of the products, and thus needs to be disposed on an unsymmetrical ground plane 130. In other words, the conventional circularly polarized antenna is restricted to a certain area of the ground plane 130, and cannot be disposed on the symmetrical ground plane. Compared with the radiation pattern in FIG. 1B, when the circularly polarized antenna is placed on the unsymmetrical ground plane shown in FIG. 2B, the gain of the radiation wave in the +z direction (vertex direction) apparently decreases and impairs the reception of the antenna. That is to say, the conventional circularly polarized antenna loses the property of circular polarization when applied to the current communication products, for the configuration and position thereof are limited.